1. Field of the Invention
The invention relates to a process for producing a cutting base and, in particular, to a process for producing a cutting base intended particularly for sound recordings and including a cuttable copper layer precipitated on a substrate, with the copper layer having recrystallizing characteristics as a result of the composition of the bath, the convection and the current density in the electroplating bath.
2. Background of the Art
It is known according to German Pat. No. (DE-PS) 2,811,888 and German Laid-Open Patent Application No. (DE-OS) 3,114,131 to produce video and audio records by providing metal substrates with groove modulations. To do this, a thin coating of copper is initially applied onto a substrate of aluminum, stainless steel or the like by electroplating from a copper bath and the groove modulation is then cut electromechanically into the copper layer by means of a cutting stylus which is guided over the metal substrate.
The electroplating bath here has such dimensions that the copper layer has a Vickers hardness between 110 and 300, since optimum cuttability has resulted in this hardness range. Softer copper of a hardness of, for example, 90 HV, results in smudged recordings, while harder copper has too high a resistance to cutting. Greater hardness of the precipitated copper is attained particularly by organic baths including components such as brightener additives.
In the known process, the brightener additives and bath conditions are dimensioned to result in a hardness range of from 160 to 220 HV. This causes recrystallizable copper to be precipitated; i.e., copper which is in an unstable phase. The range of hardness of this unstable phase of the copper extends from an initial hardness predetermined by the bath to a minimum hardness value which is practically independent of the bath. In this hardness range, the copper attains optimum hardness while simultaneously being ductile and has metal removal characteristics which are a prerequisite for good cutting behavior. If during cutting of the groove modulation in such a copper cutting base, the cutting stylus is guided over the metal surface at a setting angle which is predetermined for the respective copper consistency and the desired cutting depth and which lies, for example, between 10.degree. and 20.degree., the known process results in a copper master which, for sound recordings, is clearly superior to the conventional master in the form of a lacquer recording.
It has been found that, compared to a lacquer recording, not only are there less rejects produced, but the quality of the recording is additionally improved, for example, the signal to noise ratio by about 10 dB. Moreover, phonograph records produced according to the copper cutting technique have no leading or trailing echoes which could not be avoided in the conventional lacquer recordings. Additionally, bubbles, air inclusions, shrinkage cavities or the like are practically eliminated. The production of metal positives or negatives, such as, for example, matrixes for the production of the final record carriers, no longer requires complicated silver plating. The cut record carrier (master) can be stored indefinitely and further copies can be made at any time.
However, it has been found that in the above-described process the recrystallization phase required for quality and feasibility of the cutting process employing copper precipitated on a substrate is still limited in time. This time limitation prevents such uncut copper cutting bases from being stored as desired and thus causes difficulties in studio operations because the cuttability time of the copper cutting bases after their production by electroplating is relatively short. Although the duration of the recrystallization phase has been extended by cooling of the copper cutting bases, this measure is not always sufficient.